Lanthanum borate fiber-optic glass

ABSTRACT

Glass having a high refractive index which is used as core glass of glass fibers present in a bundled form in a fiber-optic element. This glass has a composition in percent by weight between the following limits: B2O3 19-22 La2O3 34-38 A12O3 2-5 ZrO2 6-9 Nb2O5 8-15 Ta2O5 412 ThO2 8-20 BaO 6 TiO2 3

Ulla-v. Vvv'v [72] Inventors Gertraud Agnes Anna Piesslinger; [56]References Cited Hubertus Mathieu Johannes Josephus UNITED STATESPATENTS Kunnen, Emmsingel Eindhmen 2,434,149 1/1948 De Paolis 106 47 XNehe'lands 2,971,854 2/1961 Geffcken 106 47 x [211 PP 834,495 3,081,1783/1963 Weissenberg et al. 106/47 [221 Flled June 1969 3,150,990 9/1964Faulstich 106/47 [451 Patnted 1971 3,494,354 2 1970 Yokota et 81..106/52 x [731 AS51811 3,503,764 3/1970 Young 106/54 New York, N.Y. I[32] Priorities June 29, 1968 Primary ExaminerTob1as E. Levow [3 3Netherlands Assistant Exam iner- W. R. Satterfield [31 1 09259;Att0meyFrank R. Trifari Apr. 1, 1969, Netherlands, No. 6905000 ABSTRACT:Glass having a high refractive index which is used as core glass ofglass fibers present in a bundled form in a fiber-optic element. Thisglass has a composition in percent by weight between the followinglimits:

[54] LANTHANUM BORATE FIBER-OPTIC GLASS B203 19 22 3 Claims, N0 DrawingsL3203 34 3 Al O 2-5 [52] 0.8. CI Q l3056/5906, Z120; a Nb205 815 51 1m.(:1 c036 3/00, Tarzo C03c13/00 ThOZIIIIIIIIIIIIIII:IIIIIIIIIIIIIs-zo 50Field 61 Search 106/50, 47, g 52, 54;350/96,176 TiQ 3 LANTHANUM BORATEFIBER-OPTIC GLASS The invention relates to glass of a high refractiveindex which is suitable as a core of glass fibers which are bundled in afiber-optic element.

In the last years such elements have often been used where pictures of avery weak brightness must be transmitted without loss of definition dueto dispersion being allowed to occur. They are used, for example, inimage amplifiers and television camera tubes. A bundle of a large numberof fibers having a very small diameter is used in these fiber-opticelements.

The operation of such a fiber is based on the recognition of the factthat a beam of light which impinges upon one end of the fiber remains,by means of total reflection, substantially within the fiber whenpassing through it, and reaches the other end at substantially the sameintensity. To obtain this such a fiber consists of a cylindrical core oftransparent material having a high refractive index (n,) which core isconcentrically surrounded by a sleeve of glass having a low refractiveindex (n if necessary, a layer of a different material may be providedon the outer side of the sleeve so as to avoid scattered light in thefiber plate and to obtain a definition which is as high as possible. Theaim is to render the value of n, as high as possible and that of n assmall as possible in order that the critical angle 6 is as large aspossible, which is the angle formed by the beam of light impinging uponthe fiber-optic element and the normal on the end face of the fiber. Therelation: n sin= /n n exists between this critical angle, the refractiveindices of the two types of glass and the refractive index of theambient medium (n It is to be noted that in this respect cylindrical isunderstood to mean any closed shape, thus not only circular, but alsorectangular or polygonal.

Optical fibers are manufactured in known manner starting from a groundand possibly polished rod of a glass having a high refractive index, forexample, of a diameter of 2% cms. and a length of 25 cms., by sliding anaccurately fitting tube of a glass having a low refractive index on thisrod and by drawing out the entire unit to fibers of the desired diameterwhile using heat.

There are glasses which are technologically acceptable for use as sleeveglass, which have a refractive index from 1.48 onwards. To obtain anoptical fiber having a critical angle which is as large as possible sothat the loss of light is as small as possible, a glass is requiredwhich can be used as a core of optic fibers and has a refractive indexof at least 1.81. The choice of glasses having these high refractiveindices is impeded because all known glasses more or less tend tocrystallize. When drawing out the fibers there must be no trace ofdevitrification, because a comparatively large loss of light principallycaused by dispersion occurs as a result thereof. Glasses which may beused without objection, for example, for the manufacturer of lensesbecause they are only pressed and subsequently ground, may be unusablefor the present use, because a slight tendency to crystallization duringthe drawing operation carried out in two or three stages becomesmanifest in impermissible crystallization.

When the fiber-optic element is used in one of the abovementionedelectronic devices, compounds of certain elements may not at all bepresent or only to a given maximum (pb, Cd, Se, F, Cl and Bi) inconnection with poisoning of the photocathode.

A core glass is known which reasonably satisfies the abovementionedrequirements. This glass has a composition in mole percent within thefollowing limits:

GeO 35-62 810 [0-30 TiO, 0-25 La,0, 0-15 210, O-IO Tn,0 0-5 together atleast 10. ZnO 5-15 B: [3,0, Alp, zro, Nb,0

Ta,0; Tho, BaO no,

Glasses within the following range of composition, also in 2 percent byweight, are preferred:

BIO: Lap, 10,0, zro, M1,!)6

Ta,O, Tho, BaO TiO,

The stability of these glasses is greatest within the range describedabove because crystallization only takes place at temperatures which are50 C. higher. Due to the greater stability the processibility is stillbetter because the thermal treatment is less critical.

The invention will now be described with reference to an Example of themanufacture of a fiber-optic element.

First a glass tube was made having a wall thickness of l to lit mms., anexternal diameter of l7k mms. and a length of 300 mms. consisting ofglass having one of the following three compositions in percent byweight These glasses have a refractive index n,,=l .50, a linearcoefficient of expansion between 30 and 300 C. of 66.2-54.6 and54.9Xl0'7, respectively per C., an annealing point (this is thetemperature at which the viscosity v=l0l3.4 poises) of 540 and 487 C.,respectively, and a softening point (this is the temperature at whichthe viscosity v==107.65 poises) of 696, 697and 693 C., respectively.

A cylinder having the same length was ground therein consisting of glassof one of the compositions of the following table. This table alsostates for the relevant glasses the refractive index (n,,), the linearcoefficient of expansion between 30 and 300 C. (u.c.) the annealingpoint (HOT) and the softening point (AVP).

Composition in percent by weight 10 0. HOT(C.) 669 The combinations thusobtained were first drawn out to fibers having a diameter ofapproximately 300/11. at a tempera ture of between 770 and 780C. Thesefibers were bundled to a diameter of 12 mms. and these bundles wereagain drawn out to 300/ so that the original fiber acquired a diameterof approximately 6/}L. The composite fibers obtained were cut to lengthsof 100 mms. and were bundled in a dense pack in an.

ampul having a diameter of 25 mms. of borosilicate glass of one of thethree first-mentioned compositions. The filled ampul was evacuated,sealed and heated at a temperature of between 68O and 700 C. forone-half to 1 hour. Platelike fiber-optic elements were cut from theproduct obtained and these were polished.

What is claimed is:

1. Substantially lead-free glass having a refractive index of at least1.81 and capable of being drawn out several times at a temperaturebetween 700 and 800 C. without crystallizing and suitable as a core ofglass fibers which are bundled in a fiber-optic element, said glassconsisting essentially of a composition between the following limits inpercent by weight:

2. Glass as claimed in claim 1, characterized in that it has acomposition between the range limited as follows:

8,0, 19-21 Tap 4-10 Lap, 34-38 no, 840 A1103 M0 5 6 210, 6-9 no, 2 3 wo, was

3. Glass as claimed in claim 1, characterized in that it has acomposition within the range limited as follows:

8,0, 19-22 Ta,0 l0-l2 La,0, 34-38 no, 840 mp, 2-5 Ba() 5 6 210, 6-9 Tao,s 3 mp, 21-10 392?? UNITED STATES EATENT OFFICE CERTIFICATE OFCORRECTi'ON Patent N H Dated October 26,

Inventor(s) G rtraud A.A. Piesslinger and Hubertus M.J.J. Kunnen It iscertified that error appears in the aboveidentified patent and that saidLetters Patent are hereby corrected as shown below:

Column 2, line 53, v.-1o13.4" should be 7 =10 line 54, "540 and 487C"should be 540 and 516 and 487 C,-

line 55, T 107.65" should be 7 =10 Signed and sealed this 2nd of May1972.

(SEAL) Attest:

EDWARD MQFLETCHER, JR. ROBERT GOTTSCHALK Attest'ing Officer Commissionerof Patents

2. Glass as claimed in claim 1, characterized in that it has acomposition between the range limited as follows: B2O319-21 Ta2O5 4-10La2O3 34-38 ThO2 8-20 Al2O3 2-4 BaO 6 ZrO2 6-9 TiO2 3 Nb2O5 10-15 3.Glass as claimed in claim 1, characterized in that it has a compositionwithin the range limited as follows: B2O3 19-22 Ta2O5 10-12 La2O3 34-38ThO2 8-20 Al2O3 2-5 BaO 6 ZrO2 6-9 TiO2 3 Nb2O5 8-10